1. Field of the Invention
The present invention relates to ultrasonic motors and, more particularly, to an ultrasonic motor which uses a laminated piezoelectric body as a drive source.
2. Related Art and Prior Art Statement
In recent years, attention is paid to the ultrasonic motor as a new motor substituted for an electromagnetic motor. The ultrasonic motor has the following advantages as compared with a conventional electromagnetic motor:
(1) Low-speed and a high-thrust force are produced without gears; PA0 (2) A retaining force is large or high; PA0 (3) Stroke is long, and resolution is high; PA0 (4) Silence is rich; and PA0 (5) Magnetic noises are not generated, and affection or influence of the noises does not also suffer.
As a conventional ultrasonic linear motor, there is one disclosed in Literature: "Paper Feed Device Due To Planar-Plate Motor Utilizing Longitudinal-Bending Multiple Mode Vibrator or Oscillator" (Yoshiro TOMIKAWA et al, National Conference Of Electronic Information Communication Society, Spring 1988, P146). As shown in FIG. 20 of the attached drawings, the oscillator in the ultrasonic linear motor is arranged such that three (3) piezoelectric ceramics 202, 203 and 204 are bonded or adhered to a resilient or elastic body 201 which is formed by a stainless steel sheet or plate. The oscillator is held or retained by silicon rubber 210. The piezoelectric ceramics includes one for an L-mode (longitudinal oscillation mode) at a center and two for a B-mode (bending oscillation mode) at both ends which are bonded to each other. As a configuration in which resonant frequencies of both modes are coincident with each other, alternating voltages corresponding to the resonant frequency are applied to the piezoelectric ceramics 203 of the L-mode and the ceramics 202 and 204 of the B-mode. A phase difference between these alternating voltages is adequately provided whereby it is possible to generate ultrasonic elliptic oscillation at a portion indicated by oblique lines in FIG. 18 and FIG. 19. In order to drive paper (the body to be driven) 205, for example, if the body to be driven is arranged through a pair of pressure rolls 206 serving as an urging mechanism, by generating ultrasonic elliptic oscillation, it is possible to linearly drive the paper (the body to be driven) 205. A portion of the piezoelectric element is cut off or separated and is brought to a feedback terminal, and the feedback terminal is utilized for self-excited oscillation. Further, signals from two locations on the piezoelectric element are connected in parallel to each other so as to be brought to feedback signals, to detect oscillation of only a third overtone, and so as to be brought to a feedback terminal so that the feedback terminal is utilized for self-excited oscillation.
Moreover, as a conventional ultrasonic linear motor, there is one disclosed in Japanese Patent Laid-Open No. SHO 63-277477 (277477/1988). Hereinafter, the ultrasonic linear motor will be described with reference to the publication. In FIG. 1 of the publication, a piezoelectric element 2a (or 2b) is provided on a substrate 1 to excite quaternary bending oscillation, and a piezoelectric element 2c is used to excite primary longitudinal oscillation. Resonant frequencies of these respective oscillations are coincident with each other. The oscillator is arranged at positions of nodes of the bending oscillation, As shown in FIG. 4 of the publication, an urging spring is used to urge a slider. If alternating voltage coincident with the resonant frequency is applied to the piezoelectric element 2a and the piezoelectric element 2c, the slider moves in a right-hand direction in the figure. When alternating voltage coincident with the resonant frequency is applied to the piezoelectric element 2b and the piezoelectric element 2c, the slider moves in a left-hand direction in the figure. Furthermore, a feedback terminal is taken out from a portion of the piezoelectric element to form a self-excited circuit.
Further, as a conventional ultrasonic linear motor, there is one disclosed in Japanese Patent Laid-Open No. HEI 1-126176 (126176/1989). The ultrasonic linear motor will hereunder be described with reference to a publication. In FIG. 1 of the publication, a piezoelectric body is arranged between projections which are provided in a longitudinal direction of an oscillation plate, to excite a primary longitudinal oscillation mode in the longitudinal direction by the piezoelectric element. The piezoelectric body is arranged between the projections which are provided laterally of the oscillation plate, to excite a primary width bending oscillation mode in a lateral direction by the piezoelectric element. A body to be driven (paper) urged to a predetermined position on the oscillator is fed by a roller.
However, the conventional ultrasonic linear motors have the following problems. First, problems of the ultrasonic motor disclosed in the literature: "Paper Feed Device Due To Planar-Plate Motor Utilizing Longitudinal-Bending Multiple Mode Vibrator" (Yoshiro TOMIKAWA et al, National Conference Of Electronic Information Communication Society, Spring 1988, P146) will be described. Three piezoelectric ceramics (thickness: 0.5 mm) are bonded to an elastic body. Piezoelectric effects of the piezoelectric ceramics are utilized so that oscillation is excited. Generally, an electric-mechanical bonding coefficient in the piezoelectric lateral effects of the piezoelectric ceramics is of the order of substantially 30%.about.40%, and is considerably low as compared with the fact that the electric-mechanical bonding coefficient in the piezoelectric longitudinal effects is substantially 60%.about.70%. If the electric-mechanical bonding coefficient is low, input voltage muse be raised in order to generate oscillation of the same level. Since the piezoelectric element is also a plate or sheet material whose thickness is 0.5 mm, if the input voltage is not also raised, it is impossible to take out a large or high output. In practice, there occurs a problem that relatively high voltage of the order of several tens of +V ms to 100 V ms is required. Moreover, there also occurs a problem that, if an oscillation level is raised, adhesion between the elastic body and the piezoelectric element is peeled off.
Next, a problem of an arrangement disclosed in Japanese Patent Laid-Open No. SHO 63-277477 (277477/1988) as a conventional ultrasonic linear motor will be described. Three piezoelectric ceramics (thickness: 0.6 mm) are bonded to a resilient or elastic body. Piezoelectric lateral effects of the piezoelectric ceramics are utilized so that oscillation is excited, similar to the aforesaid example. For this reason, an electric-mechanical bonding coefficient is reduced so that input voltage must be raised. There is a problem that, since a piezoelectric element is also a plate or sheet material whose thickness is 0.6 mm, a large or high output cannot be taken out if input voltage is also raised. Moreover, there occurs a problem similar to the aforesaid example that, if an oscillation level is raised, bonding or adhesion between the elastic body and the piezoelectric element is peeled off. Furthermore, the present example has the following problem. That is, in order to change over a drive direction of a body to be driven, voltage applied to the piezoelectric element is changed over. Accordingly, since one of the three piezoelectric elements is not used, it is impossible to sufficiently produce a power. Regarding a method in which a signal from the piezoelectric element for detection of oscillation is used to perform feedback, feedback is merely performed by a signal in which the longitudinal oscillation and the bending oscillation are superimposed upon each other or a signal of a third overtone (bending oscillation), and oscillation that is longitudinal oscillation is not considered. Further, there is no clear or definite description in the specification, regarding the bonding position of the piezoelectric element.
Problems of an arrangement disclosed in Japanese Patent Laid-Open No. HEI 1-126176 (126176/1989) as the conventional linear motor will next be described. A problem that bonding is peeled off because the piezoelectric element is provided between the projections on the elastic body is solved. However, Judging from the before and behind relation although there is no clear or definite description in the specification, it has been considered that the piezoelectric lateral effects of the piezoelectric ceramics are utilized so that oscillation is excited, similar to the aforementioned example and the last example. For this reason, there is a problem that the electric-mechanical bonding coefficient is reduced so that the input voltage must be raised. Furthermore, there are problems that, since, in the present example, the primary longitudinal oscillation mode in the longitudinal direction and the primary lateral bending oscillation mode in the lateral direction are utilized, piezoelectric elements different in size from each other must be prepared as the piezoelectric element as shown in FIG. 1 of the publication, and that shape or configuration as the ultrasonic oscillator becomes unbalanced so that a mode superior in symmetrical property or symmetrizing cannot be excited.